Simplify handling of tuple list (ordering, aggregations)

This:

```
p.sort([["firstname", "asc"]])
```

looks unintuitive, despite being correct.

Allow:

```
p.sort("firstname")
```

This would be nice to have, but should not be allowed as it is ambiguous:

```
p.sort(["firstname", "asc"])
```

Does it mean to sort `firstname` ascending or it means sort by fields `firstname` and `asc` in default (ascending) order?
